If you area good planner (I am). The $49 threshold and 7-10 day shipping is an absolute non issue. Just gotta stay ahead of the curve so urgency of receipt isn't a big deal.
Congrats on your expert planning and never having life rear its ugly head and screwing with that plan.
For those of us that don't yet have that part down, Prime is a great option. We have free same day in Dallas and a few weeks ago our vacuum broke unexpectedly. We have 2 dogs that shed like there is no tomorrow and can't go more than a couple days or our house gets pretty messy. It was a Saturday morning, vacuum wouldn't work, hopped on Prime and within about 10 hours, we had a brand new one at the door.
Once I get to the point of my plan always working out, I will be in a position to drop Prime. Let the countdown begin!
Really though, its cheap enough to save plenty of money over the long run and has a lot of other perks.